The present invention relates to a method for producing a compensator for liquid crystal display useful in cancelling the coloration of a liquid crystal display and obtaining a black-and-white display.
Liquid crystal display occupies an important position in the display field because of such characteristic features as being driven at a low voltage, light weight and low cost. For example, a super-twisted nematic (hereinafter referred to simply as "STN") liquid crystal display of a simple matrix driven type permits a large screen display in a multiplex driven dot-matrix system, and as compared with the conventional twisted nematic (TN) type liquid crystal display, the STN liquid crystal display is characteristic in that the contrast is high and the viewing angle is wide. Therefore, the STN liquid crystal display is widely used in the field of liquid crystal display which requires a large-screen display such as, for example, personal computers, word processors and various data terminals. However, since the STN type makes display on the basis of a birefringence effect, coloration such as yellow or blue has been unavoidable. Such a colored mode display is not only undesirable to the users but also involves a serious drawback of being unable to cope with coloring of a display unit.
Also in a liquid crystal display using a thin film transistor (hereinafter referred to simply as "TFT") which is a typical example of the active matrix driven type, there arises, in addition to the difficulty of manufacture, a serious drawback of coloration when the cell gap is thinned for the purpose of improving the response speed or improving the viewing angle characteristic.
For changing the colored mode into the black-and-white mode there has been practically adopted a so-called double-cell method wherein compensation is made by disposing on an STN liquid crystal cell for display another liquid crystal cell for compensation having the same cell gap and a reversed twist angle. For a simpler method, compensating films (phase films) such as stretched polycarbonate films adjusted only in birefringence characteristic in conformity with a display cell have been developed and some of them have been put to practical use.
Although the double-cell method can realize a black-and-white display of a fairly high grade, it involves serious drawbacks such as high cost, heavy weight and large thickness. On the other hand, the phase film compensation method has merits such as being inexpensive, light weight and thin, but the film used therein is a uniaxially stretched film not having a twist structure, so the compensation capable of being made by this method is only of phase difference and not of optical rotatory dispersion. Thus, the compensating performance, which is important, is unsatisfactory and there merely is obtained a pseudo-black-and-white display. Also as to contrast, it is fairly lower than that obtained in the double-cell method.
To compensate for such drawbacks of both methods, the present inventors have previously proposed a compensator for liquid crystal display constituted by a liquid crystalline polymer film with a twisted nematic structure solidified (Japanese Patent Laid Open No.150559/1989). This proposed compensator was an epoch-making compensator wherein an equal compensating performance to that attained by the double-cell method was imparted to a single film formed on a light transmitting base with alignment film. However, this light transmitting base with alignment film is required to have heat resistance and solvent resistance because at the time of production it also plays the role of orientating the liquid crystalline polymer. The said light transmitting base is further required to be transparent and optically isotropic because it is also used as a constituent of the compensator. Thus, in the conventional methods, requirements for the light transmitting base are strict and so the range of selection of such light transmitting base is narrowed. Consequently, the compensator performance and the product form have been limited.